The origin of tonsteins, an overview, and links with seatearths, fireclays and fragmental clay rocks

Tonsteins are volcanic ash falls in coal-bearing sequences that have altered to kaolinite. Only in the last twenty years or so has the volcanic origin become firmly established. The evidence for the volcanic origin encompasses bed form, including lateral extent, structures, textures, volcanogenic mineralogy and geochemistry. The lines of evidence are reviewed from a historical perspective. Tonsteins came to prominence over a hundred years ago because of their stratigraphic value. Tonsteins continue to be of value in coalfield exploration, but their stratigraphic value has been enhanced in recent years with the ability to determine radiometric ages with a high level of accuracy. The geochemistry not only provides good evidence for the volcanic origin, but also enables tectomagmatic deductions to be made for areas external to the coal basin. In addition, some tonsteins are the indicators for the discovering of rare metal ore deposits (Nb, REEs, and Ga). There is also the potential to use the geochemistry to identify specific tonsteins; essentially a chemostratigraphic approach. Tonsteins are seen as one class of bentonites; others include K-bentonites, metatonsteins, and possibly illitic-bentonites.Tonsteins have been linked to seatearths, fireclays and fragmental clay rocks. Although all such rocks contain kaolinite, the origins differ. The seatearths contain rootlets and were subjected to pedogenic activity. Alteration of the silicate minerals is limited. Under conditions of a high water table, and in the presence of organic matter, reducing conditions prevailed leading to gleization. On the other hand, the fireclays show extensive alteration to kaolinite. There is a link with the major marine bands and this has implications in terms of sequence stratigraphy. The fragmental clayrocks have also been linked to soil-forming processes, but in this case possibly the reworked soil was the product of an open system containing free alumina minerals that reacted with silica in solution during diagenesis.

► Tonsteins are kaolinite rich, altered volcanic ash falls in coal-bearing sequences.
► Evidence for their origin is reviewed from a historical perspective.
► Nomenclature is reviewed, including relationship with bentonites.
► Terminology, such as rhyolitic tonstein, would reflect the ash composition determined from the geochemistry.
► Although differing in origin fireclays have sequence stratigraphic implications.